The wave of popular protest in Egypt and Tunisia has reached the streets of Iran, where police on Monday used teargas and paintballs to disperse anti-government protesters.
There are reports of at least one death and several other people have been injured after people marched near Imam Hossein square in defiance of a ban.
The opposition Green Movement organised the rally in support of the protests in Tunisia and Egypt.
The fiercest clashes were reported on Azerbaijan Street, close to Azadi Square. Riot police remained deployed in central Tehran as night fell and the protesters dispersed.
The BBC also received reports of similar protests being held in the cities of Isfahan, Mashhad and Shiraz.
Earlier, access was blocked to the homes of two prominent opposition leaders and telephone lines were cut.
Mir Hossein Mousavi and Mehdi Karroubi, a former Speaker of parliament and senior cleric, are being held under house arrest.
The BBC reports both men disputed the re-election of President Mahmoud Ahmadinejad in June 2009, which triggered mass protests that led to a brutal crackdown by the regime.
The opposition says more than 80 of its supporters were killed over the next six months - a figure the government disputes. Several have been sentenced to death and dozens jailed.
Mr Mousavi's website Kaleme.com said that according to unconfirmed reports, "hundreds of protesters" had been arrested on Monday.
There has been no official confirmation, but witnesses told the BBC that dozens had been taken away in police vans from the area.
US Secretary of State Hillary Clinton says Iran should heed the will of its people.
AFP reports she hailed the "courage" and "aspirations" of anti-government protests in Iran and pressed Tehran to follow Egypt's example and "open up" its political system.
Arab turmoil
Other parts of the Arab world are also in turmoil. Hundreds of youths in Algeria demonstrating against the failure of government job promises have clashed with security forces in the northern town of Akbou, about 180km east of the capital Algiers.
President Abdelaziz Bouteflika came to power there in 1999.
In Yemen, more than 1000 anti-government protesters have clashed with security forces.
The crowd of mostly students were demanding the resignation of President Ali Abdullah Saleh, who has been in power since 1978. He has promised to step down in 2013.
Anti-government protesters have also clashed with police in Bahrain. Police used tear gas and rubber bullets to stop demonstrators reaching the capital Manama.